15th International Conference on Quantum Physics and LogicQPL 2018

Halifax, CanadaJune 3–7, 2018

Quantum Physics and Logic is an annual conference that
brings together researchers working on mathematical
foundations of quantum physics, quantum computing, and
related areas, with a focus on structural perspectives and
the use of logical tools, ordered algebraic and
category-theoretic structures, formal languages,
semantical methods, and other computer science techniques
applied to the study of physical behaviour in
general. Work that applies structures and methods inspired
by quantum theory to other fields (including computer
science) is also welcome.

QPL 2018 was held at Dalhousie University
from June 3–7, 2018. It was
co-located with MFPS
2018, which takes place from June
6–9. The two conferences had a joint
special session and a joint invited speaker.

Invited speakers and invited tutorials

Invited speakers

In this talk I will give a gentle introduction to
quantum surface codes, and to how the ZX calculus
may be used as a high-level design and compilation
language for them. I will show how using the ZX
calculus simplifies an understanding of how the
codes work, and in particular gives immediate access
to the connection between 2D and 3D codes. Looking
particularly at the operations of lattice surgery, I
show how they form a direct model for the generators
of the calculus in terms of 'splitting' and
'merging' operations. Interpreting the calculus in
this operational way means coming to grips with
probabilistic procedures, and I look at ways we can
deal with this within ZX. Finally I show how having
the calculus as a graphial language for error
correction pays off in the compilation problem for
quantum processes, and show how we get new
operational procedures for protocols, including
magic state distillation, from equational rewriting
of the ZX diagrams.

Debbie Leung (University of Waterloo):
"Embezzlement-based nonlocal game that cannot be
played optimally with finite amount of entanglement"[abstract]

We first summarize recently found nonlocal games
(with finite number of classical questions and
answers) whose optimal winning probability can only
be attained as a limit of strategies using
arbitrarily high dimensional entangled states.

Then, we focus on one such game, an explicit
three-player game with very few classical questions
and answers. This game is based on the coherent
state exchange game introduced
in arXiv:0804.4118,
which in turns is based on embezzlement of
entanglement due to van Dam and Hayden. We discuss
the main ideas behind each of these ingredients, and
how they can be put together to obtain a
quantitative tradeoff in the winning probability vs
the dimension of the entangled state shared by the
players.

Quipper is a functional programming language for
quantum computation which has been used to implement
various elaborate quantum algorithms. At the moment,
Quipper is implemented as an embedded language,
whose host language is Haskell. One of the
disadvantages to Quipper being an embedded language
is that the Haskell type system, while providing
many type-safety properties, is not in general
strong enough to ensure the full type-safety of
quantum programs.

The Proto-Quipper language provides a foundation for
the development of a stand-alone (i.e.,
non-embedded) version of Quipper. Like Quipper,
Proto-Quipper acts as a circuit description language
and provides the ability to treat circuits as data
in order to manipulate them as a whole. However,
Proto-Quipper is also designed to “enforce the
physics”, in the sense that its type system would
detect, at compile-time, programming errors that
could lead to ill-formed or undefined circuits.

I will start my talk with a brief introduction to
quantum computing and a discussion of the design
principles behind Quipper. I will then present
Proto-Quipper and discuss open problems.

Many experiments in the field of quantum foundations
seek to adjudicate between quantum theory and
speculative alternatives to it. This requires one to
analyze the experimental data in a manner that does
not presume the correctness of the quantum
formalism. The mathematical framework of generalized
probabilistic theories (GPTs) provides a means of
doing so. We present a scheme for determining what
GPTs are consistent with a given set of experimental
data. It proceeds by performing tomography on the
preparations and measurements in a self-consistent
manner, i.e., without presuming a prior
characterization of either. We illustrate the scheme
by analyzing experimental data for a large set of
preparations and measurements on the polarization
degree of freedom of a single photon. We find that
the smallest and largest GPT state spaces consistent
with our data are a pair of polytopes, each
approximating the shape of the Bloch Sphere and
having a volume ratio of 0.977 ± 0.001, which
provides a quantitative bound on the scope for
deviations from quantum theory. We also demonstrate
how our scheme can be used to bound the extent to
which nature might be more nonlocal than quantum
theory predicts, as well as the extent to which it
might be more or less contextual. Specifically, we
find that the maximal violation of the CHSH
inequality can be at most 1.3% ± 0.1 greater than
the quantum prediction, and the maximal violation of
a particular noncontextuality inequality cannot
differ from the quantum prediction by more than this
factor on either side.

Tutorials

Ciáran Lee (University College London):
"Computation in a general physical setting"[abstract]

The advent of quantum computing has challenged
classical conceptions of which problems are
efficiently solvable in our physical world. This
raises the general question of what broad
relationships exist between physical principles and
computation. This tutorial will explore this
question in the the operationally-defined framework
of generalised probabilistic theories. To begin, the
limits on computational power imposed by simple
physical principles will be explored. After this, we
shall investigate whether post-quantum interference
is a resource for post-quantum computation. The
tutorial will end with a conjecture asking whether
quantum theory is optimal for computation in the
landscape of generalised probabilistic theories
satisfying a single physical principle.

Conference program

Talks began at 9am on Sunday morning, June 3, and ended at
5:30pm on Thursday afternoon, June 7. The poster session
was on Monday evening, and the conference dinner was on
Wednesday evening.

Best student paper award

At each QPL conference, an award for the best student
paper is given at the discretion of the program
committee. Papers eligible for the award are those where
all the authors are students at the time of submission.

This year's best student paper award went to Matthew
Amy for the paper "Towards large-scale functional
verification of universal quantum circuits".

Travel to Canada

Most foreign nationals now need an electronic travel
authorization before flying to Canada. This applies to
all visa-exempt foreign nationals (for example, European
citizens). It only costs $7 and can be done efficiently at
the eTA website.
The eTA requirement does not apply to U.S. citizens, nor to
visitors who need an actual visa to travel to Canada.

Registration and local information

Registration is now closed. The registration fees were as
follows:

Regular participants: $110 (MFPS only) or $130 (MFPS and QPL).

Students: $70 (MFPS only) or $90 (MFPS and QPL).

Before May 15, there was an early registration
discount of $10.

There was a joint QPL/MFPS conference dinner on
Wednesday, June 6. The dinner took place
at the University Club, 6259 Alumni Crescent (building
C440 on the campus map).

All QPL talks were held in room 1020 of
the Kenneth C. Rowe Management building at 6100
University Avenue (building E260 on
the campus map). Here is a
map of the area:

For those arriving on or before Saturday, there was an
informal gathering at the Henry House Pub, 1222
Barrington Street (in the pub in the basement, not
the formal dining room upstairs) on Saturday, June
2, starting at 6pm.

Student support

Graduate student participation is encouraged at QPL.
Students will pay a reduced registration fee. We were also
able to provide limited support for travel and
accommodations to some students. If you are interested in
this, please send a request
to selinger@mathstat.dal.ca
by April 30. Please also arrange for a letter of
reference from your supervisor, or appropriate other
person, to the same email address, explaining whether the
student has access to funding from local sources and how
much.

Submissions

Prospective speakers are invited to submit one (or more) of the
following:

Original contributions consist of a 5-12 page
extended abstract that provides sufficient evidence of
results of genuine interest and enough detail to allow
the program committee to assess the merits of the
work. Submissions of works in progress are encouraged
but must be more substantial than a research proposal.

Extended abstracts describing work
submitted/published elsewhere will also be considered,
provided the work is recent and relevant to the
conference. These consist of a 3 page description and
should include a link to a separate published paper or
preprint.